Lubricant oil is used in various fields in which friction decrease is required. In old times, natural fat and oil, petroleum purified products, and others were used. In recent years, however, synthetized lubricant oils have come to be synthesized and used in accordance with articles to be used. In particular, synthesized esters are excellent in thermal stability. Specific examples thereof include organic acid esters, phosphoric acid esters, and silicic acid esters.
Out of the organic acid esters, polyol esters (condensed esters each made from a polyhydric alcohol and a carboxylic acid) are used since the esters 1) are low in pour point and high in viscosity index to be wide in a use-temperature range thereof, 2) are high in flash point and small in evaporation quantity, 3) are excellent in thermal and anti-oxidization stabilities, 4) are good in lubricity, 5) have cleaning and dispersing effects, and 6) have biodegradability. In many fields, in particular, hindered esters are used since the esters are in thermal and anti-oxidization stabilities.
However, in recent years, with developments of industrial techniques, high productivity and operation stability have been constantly required; thus, lubricant oils have come to be required to be higher in endurance and higher in heat resistance.
For example, Patent Document 1 discloses a high-temperature stable lubricant which is economical, is resistant against thermal decomposition, and is smaller in viscosity increment than the existing lubricants. This lubricant is suitable, particularly, for gas turbine engines derived from aircrafts. This lubricant includes a mixed polyol ester in which a carboxylic acid part of this ester includes (a) 2 to 40% by mole of an aromatic carboxylic acid and (b) 60 to 98% by mole of a C5 to C20 aliphatic carboxylic acid, and an alcohol part of the ester includes an aliphatic polyol.
Patent Document 2 discloses a synthesized ester base stock including a reaction product made from pentaerythritol for industries, and a carboxylic acid mixture. The carboxylic acid mixture includes (1) at least one C8-C10 carboxylic acid having 6 or less reactive hydrogen atoms, (2) at least one C5-C7 carboxylic acid having 6 or less reactive hydrogen atoms, and (3) at least one C6-C10 carboxylic acid having 6 or more reactive hydrogen atoms. This synthesized ester base stock is useful for producing turbo oil for aircrafts, and has an effect of restraining the production of deposits in a turbine engine for aircrafts.
Patent Document 3 discloses a synthesized polyol ester which consists substantially of a neutral esterified product made from a specified polyol compound selected as a hydroxyl component, and a specified mono-carboxylic acid and/or a poly-carboxylic acid as one or more acid components, and which is used to produce a temperature-stable lubricant oil dispersant and/or a lubricant grease.
Patent Document 4 discloses a lubricant composition suitable for being used at high temperatures about which the evaporation amount thereof is restrained at high temperatures and the fluidity thereof is kept over a long term. This lubricant composition makes use of a lubricant base oil including 50% or more by mass of a mixture including a hindered ester compound and an aromatic ester compound at a ratio by mass of 5/95 to 95/5.
Patent Document 5 describes an a ester made from an alicyclic carboxylic acid, such as hexahydrobenzoic acid, and a specified polyhydric alcohol. The document discloses that this a ester is used, as a lubricant oil for engines or turbines, in the form of a mixture with another ester or mineral oil lubricant.